This invention concerns a thermoplastic elastomer composition which has excellent mechanical strength, thermal stability, moldability, gas-impermeability, and damping characteristics. (Conventional Techniques)
Thermoplastic elastomers are widely used as materials intermediate between rubber and plastics, or as rubber substitutes in automotive parts and others.
Various suggestions have been made traditionally concerning such thermoplastic elastomers. For example, Open Patent No. 1978-34210 discloses a partially-hardened thermoplastic blended material consisting of mono-olefin copolymer rubber and a polyolefin plastic. The mono-olefin copolymer rubber is an elastomer such as ethylene-propylene copolymer (EPM) or ethylene-propylene nonconjugated-diene polymer (EPDM), etc., while the polyolefin plastic is or polyethylene, polypropylene, etc. Partial hardening occurs under dynamic conditions, namely during mastication. Further, Open Patent No. 1979-2662 discloses a method of manufacturing a thermoplastic elastomer which consists of 40-90 parts by weight of mono-olefin copolymer rubber and 60-10 parts by weight of crystalline polypropylene and/or crystalline ethylene-propylene block copolymer, which are mixed and partially vulcanized while the mixture is still molten. In addition, Open Patent Report No. 1980-18448 discloses a thermoplastic elastomer composition which contains about 25-85 parts by weight of crystalline thermoplastic polyolefin resin, about 75-15 parts by weight of a vulcanized mono-olefin copolymer rubber blend, and 0-300 parts by weight of extender oil per 100 parts by weight rubber, and others.
However, compositions consisting of a polyolefin resin and partially vulcanized mono-olefin copolymer rubbers have insufficient moldability and long-term thermal stability, although such characteristics as tensile strength, etc., have been improved. Several suggestions have been made to solve to remaining problems: a thermoplastic elastomer composition consisting of 5-40 wt. % polyisobutylene and/or isobutylene-isoprene copolymer rubber, 30-90 wt. % ethylene-propylene copolymer rubber and/or ethylene-propylene-nonconjugated-diene terpolymer rubber, and 5-40 wt. % of polyolefin resin (Open Patent No. 1979-23702); and a method of manufacturing a partially-vulcanized thermoplastic elastomer composition characterized by dynamic thermal processing, in the presence of organic peroxide, of a mixture consisting of (a) 90-40 parts by weight of peroxide-bridged olefin copolymer, (b) 10-60 parts by weight of peroxide-dissolved olefinic plastic, (where (a)+(b) is 100 parts by weight), (c) a peroxide-unbridged carbohydrate material having a rubbery consistency, and/or (d) 5-100 parts by weight of a process oil type plasticizer (Open Patent No. 1981-15741). Typically, the peroxide-bridged olefin copolymer rubber (a) is ethylene-propylene copolymer rubber or ethylene-propylene-nonconjugated diene copolymer rubber, while peroxide-dissolved olefinic plastic (b) is isotactic polypropylene, and the peroxide-unbridged carbohydrate material having a rubbery consistency (c) is polyisobutylene or butyl rubber.
However, peroxide-bridged olefin copolymer rubber, such as ethylene propylene copolymer rubber and ethylene-propylene-nonconjugated-diene copolymer rubber, are vulcanized in the compositions thus obtained, while polyisobutylene and butyl rubber are not vulcanized; this causes molecular fragmentation. Consequently, these compositions are weak in mechanical strength, inferior in moldability, and have inadequate permanent compressive set and heat resistance.
In light of the above, it has previously been suggested using a thermoplastic elastomer composition which uses a metallic oxide and/or chloride instead of organic peroxide as a bridging agent, and which consists of (A) 10-90 parts by weight of crystalline polypropylene, (B) 10-90 parts by weight of halogenated butyl rubber (where (B)+(B)=100 parts by weight, (C) 10-120 parts by weight of olefin polymer rubber, and (D) 1-90 parts by weight of denatured polyolefin, which are partially vulcanized in the present of a bridging agent consisting of a metallic oxide and/or metallic chloride (Open Patent No. 1986-291639), or a thermoplastic elastomer composition which is partially vulcanized in the presence of a bridging agent made of a metallic oxide and/or a metallic chloride, and which consists of (A) 10-90 parts by weight of crystalline polypropylene, (B) 10-90 parts by weight of halogenated butyl rubber (where (A)+(B)=100 parts by weight, (C) 10-120 parts by weight of olefin polymer rubber, (D) 1-90 parts by weight of denatured polyolefin, and (E) 2-50 parts by weight of nitrile rubber (Open Patent No. 1986-291638). Chlorinated butyl rubber is used in both cases as the halogenated butyl rubber. In such thermoplastic elastomer compositions, only the chlorinated butyl rubber is bridged with ZnO or ZnC12. In order to improve further the mechanical strength and other characteristics, it is necessary to bridge the olefin copolymer rubber such as EPDM, and it becomes necessary to use an organic peroxide in order to accomplish this. However, we have discovered that chlorinated butyl rubber undergoes molecular fragmentation when organic peroxide is used, thereby considerably reducing its mechanical strength, moldability, and plasticity.
Therefore, an objective of this invention is to provide a thermoplastic elastomer composition which has improved mechanical strength, flexibility, moldability, long-term heat resistance, gas impermeability, damping, etc.
A particular objective of this invention is to provide a thermoplastic elastomer composition containing new components consisting of bridged olefin copolymer rubber, in order to improve the above properties.